1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, particularly to improvement of a technology of more stably reproducing intermediate gradation.
2. Description of the Related Art
Conventionally, electrophotography is adopted as a process of forming an image on a paper medium in an image forming apparatus such as a copying machine, a printer, a facsimile, and a multifunction peripheral. In the electrophotography, an electrostatic latent image is formed on a photosensitive body (typically, a photosensitive drum or a photosensitive belt) using an exposure device, and the image is formed by developing the electrostatic latent image.
Recently, high resolution of the electrophotography makes progress. For example, the exposing device has been improved to enhance the resolution of the electrostatic latent image to 2400 dpi (dots per inch).
On the other hand, a demand for improving process stability also arises with the improvement of the resolution. Because it is said that the improvement of the resolution is contradictory to the process stability, there is an important technical problem in that the stability is maintained while the resolution is enhanced. The process stability affects the finish of intermediate gradation.
Regarding a technique for achieving the process stability, Japanese Laid-Open Patent Publication No. 05-161013 for example discloses a digital recording apparatus that detects with a sensor a pattern in which a high density and a low density are repeated and modulates the density of image data in order to keep the stability of the image quality.
Further, Japanese Laid-Open Patent Publication No. 05-328112 discloses a dither process method in which the image having gradation can be restored by performing dither process corresponding to a density state of the image around each pixel constituting a gray scale image even if the target gray scale image has a deviation of the density.
Generally, in the electrophotographic image forming apparatus, the intermediate gradation is reproduced using a halftone technique. In the halftone technique, an objective gradation value is reproduced by controlling a coloring amount (typically, toner adhesion amount) per unit area using a pattern including small dots (dot pattern) or lines (line pattern). In the control of the coloring amount per unit area, plural screens are previously prepared while correlated with plural gradation values, and the screen is selected according to the density to be reproduced. In the general screen, “adhesion regions” that should be colored and “non-adhesion regions” that should not be colored are regularly disposed at a predetermined period.
Electrophotography, however, is not adequately able to form an image of extremely thin lines or small gaps due to its characteristics as shown in FIGS. 8 and 9. It is therefore said that an increased number of lines within a screen with the purpose of improving the resolution will result in deteriorated image stability.
FIGS. 8 and 9 schematically illustrate states in which the image reproducibility is degraded in the electrophotography. FIGS. 8 and 9 are sectional views schematically illustrating media to which the toner image is fixed. However, the size of media is not matched with the actual size.
As illustrated in FIG. 8, it is considered that a linear toner image has a predetermined width in the main-scanning direction or the sub-scanning direction. When the latent image used to form the toner image has a certain level of width, because the electric field between the latent image and the development roller is maintained in a constant direction even if wraparound of a line of electric force is generated by an edge effect, the toner image is stably formed to some extent. Therefore, the stable development can be performed during the development of the charged toner. On the other hand, when the latent image has the small width, the wraparound of the line of electric force becomes prominent by the edge effect on the development region, and the direction of the electric field tends to become instable. Therefore, it is difficult that the toner stably adheres to the narrow region. In the fixing, when the toner image has the large width to some extent, because unification of the toner is generated, the toner is stably fixed to media. On the other hand, for the thin toner image, occasionally the toner cannot stably be fixed to media due to diffusion of the toner. In such cases, the line looks like the state in which the line is cut, or the line cannot be reproduced at all.
As illustrated in FIG. 9, it is considered that the toner image has a gap (where the toner should not exists) having a predetermined width in the main-scanning direction or the sub-scanning direction. When the gap of the toner has a certain level of width, the gap can be maintained even if the gap is affected by the adjacent toner. On the other hand, for the narrow gap, occasionally the gap is filled by the diffusion of the adjacent toner.
FIG. 10 is an image formed actually by electrophotography from image data in which thin lines and whitened lines have different thicknesses. From the left side toward the right side, the thin lines and whitened lines increase in thickness. It is seen from this drawing that electrophotography is not adequately able to express extremely thin lines or small gaps.
Thus, even if an image forming apparatus has an exposing device with which a latent image of a resolution of 2400 dpi can be formed, formation of a latent image of the resolution of 2400 dpi is actually restricted to characters, and a halftone portion representing intermediate gradation is expressed with the number of screen lines substantially identical to that of a machine with a resolution of 600 dpi, to thereby prevent deterioration of the image stability.
Regarding the above-described method, while the reproducibility of characters is improved when the resolution of the image forming apparatus is enhanced to 2400 dpi, the number of screen lines of the halftone portion representing intermediate gradation is still identical to that in the case of the conventional resolution of 600 dpi. Namely, even if the resolution of the apparatus itself is enhanced, the number of screen lines of a halftone portion is still difficult to increase.